The monoesterified phosphate groups of diphosphoinositide and triphosphoinositide have the fastest turnover rates of all lipid components. It is generally assumed that they are involved in cation transport mechanisms. In brain, the lipids are mainly associated with myelin. Since the metabolism of myelin is generally sluggish because of the inaccessibility of its constituents, the reactive polyphosphoinositides, and the respective kinases and monophosphoesterases, may be suspected to reside in myelin-appurtenant structures such as internal or external mesaxon or perinodal regions. However, we have found high mono- and diphosphoinositide kinase activities in vitro in three myelin subfractions which contain different proportions of myelin core and appurtenances. It appears that the inositide phosphorylation apparatus is available in the myelin core, but the possibility remains that it may be dormant. We address ourselves to the problem of the location of polyphosphoinositide turnover and the character of the enzymes involved. The distribution of inositides over the myelin subfractions will be determined quantitatively, as well as the activity of the kinases and the monophosphoesterases in these fractions. Polyphosphoinositide turnover will be assessed by radioactivity-chasing experiments. Injection of phosphate into brain, and brain slice incubations, will clarify the in vivo distribution of reactive inositides over the myelin subfractions. This distribution will be verified by parallel glycoprotein labelling experiments. Polyphosphoinositide turnover will be studied in oligodendroglial cell membranes. The characteristics of the enzymes in the different myelin and glial fractions will be determined with a view of establishing their differences or identity. The results of these studies will have bearing on the question of the possible biochemical function of polyphosphoinositides.